The present invention relates to a device as defined in the preamble of claim 1 for measuring the flow of a gas or gas mixture a flow channel. Furthermore, the invention relates to a procedure as defined in the preamble of claim 9 for measuring the flow of a gas or gas mixture in a flow channel.
In prior art, a so-called Pitot tube is known, which is used to measure the flow of a gas or gas mixture in a flow channel. In a Pitot tube application, the tube comprises two sensors, one of which is used to measure the normal pressure in the flow channel and the other to measure the pressure generated in the sensor by the flow. Such sensors may be e.g. tubes, one of which has been bent against the direction of flow so that the flow enters the tube in a substantially perpendicular direction while the other is oriented in the direction of flow and measures the internal pressure in the flow channel. The pressure difference thus obtained is proportional to the square of the prevailing flow.
Where the gas or gas mixture to be measured contains large particles, condensation liquids and corroding substances, conventional sensors inserted from outside the flow channel are unreliable because static Pitot tubes are liable to get clogged up and gather condensation liquid.
The object of the present invention is to eliminate the drawbacks mentioned above.
A specific object of the present invention is to produce a device that it simple and cheap.
A further specific object of the invention is present a procedure in which the measuring duct, i.e. sensor, cannot be clogged up.
The device of the invention is characterised by what is presented in claim 1.
The device of the invention comprises a frame, a measuring duct and a measuring element. The device is connected to a flow channel, in which the flow of a gas or gas mixture is measured. The frame further comprises a measuring space. The measuring duct is mounted on the frame between the flow channel and the measuring space. The measuring duct is open at both ends, so that a pressure wave generated in the duct by the flow can advance into the measuring space. The measuring element is placed near the second end of the measuring duct in the measuring space and it is used to measure the pressure wave.
According to the invention, the measuring duct is disposed in a substantially perpendicular orientation relative to the direction of flow of the gas or gas mixture and so that it can rotate about its longitudinal axis. The measuring duct comprises a first orifice, which is disposed at the first end of the measuring duct and alternately in the direction of flow and against the flow to admit a changing and cyclic pressure wave into the measuring duct. The measuring duct further comprises a second orifice, disposed in that part of the measuring duct which extends into the measuring space to equalise the static pressures in the measuring space and measuring duct.
In an embodiment of the device, the diameter of the first orifice is larger than the inner diameter of the measuring duct to allow unimpeded generation of a pressure wave in the measuring duct. The orifice is preferably of a circular or oval form.
In an embodiment of the device, the device comprises a power means, preferably an electric motor, which is mounted on the frame and connected to the measuring duct, preferably by means of a V-belt, to rotate the measuring duct.
In an embodiment of the device, the measuring element is a microphone, loudspeaker or equivalent, which is used to convert the pressure wave into an electric signal. The device may comprise one or more measuring elements. For instance, in an arrangement comprising two microphones, the measuring duct may be a double tubular structure comprising two tubes one inside the other, forming two separate flow routes.
In an embodiment of the device, the measuring duct is connected to the flow channel and measuring space via airtight and watertight joints to ensure that external disturbances, e.g. air flow and splashes of liquid, will not produce errors in the measurement.
In an embodiment of the device, a protective air stream flowing into the measuring space is used so that the protective air flows into the measuring space and through the measuring space further via the measuring duct into the flow space. This prevents impurities from entering from the flow channel into the measuring duct and further into the measuring space.
In an embodiment of the device, the part of the measuring duct extending into the flow channel is provided with one or more orifices disposed at different angles relative to the radius perpendicular to the longitudinal axis of the measuring duct. In this case, cyclic pressure waves changing in different phase are passed into the measuring duct.
In an embodiment of the device, the device comprises means for the processing and shaping of an electric signal. These means may comprise a signal filter and a converter element. The signal filter filters the electric signal in a manner known in itself to eliminate any frequencies that differ from a sinusoidal signal. The converter element converts, in a manner known in itself, an analogue signal into a digital signal, which can be processed e.g. using a computer.
In the procedure of the invention, the measuring duct is rotated in a substantially perpendicular orientation to the flow, with the result that the first orifice comprised the measuring duct is alternately in the direction of flow and against the flow. Thus, a changing and cyclic pressure wave generated in the measuring duct through the orifice is passed into the measuring element, and the changing and cyclic pressure wave is converted in the measuring element into an electric signal, whereupon the flow is determined on the basis of the amplitude of the electric signal.
In an embodiment of the procedure, a sinusoidal signal is separated from the electric signal by filtering out all other signals. This produces a signal of a constant wavelength.
In an embodiment of the procedure, the measuring duct is rotated at a constant speed.
In an embodiment of the procedure, the flow is computed based on the formula v2=xcex94P, where v=flow and xcex94P=pressure difference, which is proportional to the amplitude of the electric signal.
The invention makes it possible to use the device in unclean conditions where the gas in the flow to be measured may contain e.g. large particles, condensation liquid and corroding substances. Disturbance cumulation in the measurement works continuously. The device is easy and cheap to construct and therefore also to maintain. Calibration of the measurement can always be successfully carried out in unclean conditions. The device can be used for measurements at high pressures because the measuring space can be implemented as a small and therefore pressure-resistant space.